Tetrahydrobiopterin, is the cofactor for tyrosine and tryptophan hydroxylase, which are the initial and rate-limiting enzymes in catecholamine and serotonin synthesis, respectively. Due to the important role of tetrahydrobiopterin in regulating these enzymes, the ability of tetrahydrobiopterin to enter the central nervous system of diseased patients and its therapeutic effects were studied. Since the beneficial effects of cofactor administration have been variable in the patient population, we also studied cofactor penetration into rat brain after peripheral dosing of tetrahydrobiopterin and several more lipophilic synthetic analogues of tetrahydrobiopterin with the hope of finding cofactors that could more easily enter the brain. Therapeutic benefit was achieved with tetrahydrobiopterin therapy in 4 of 10 patients with familial dystonia. In one out of three severely depressed patients tested in Switzerland, there was a marked clinical improvement after tetrahydrobiopterin therapy concomitant with an increase in the cerebrospinal fluid content of tetrahydrobiopterin and several biogenic amine metabolites. Two other depressed patients did not respond. In animal studies, more lipophilic cofactor analogues of tetrahydrobiopterin that were injected peripherally were not better than tetrahydrobiopterin itself at entering the brain. 6-methyltetrahydropterin, which has a shorter 6-position side chain compared to tetrahydrobiopterin, penetrated ten times better than tetrahydrobiopterin. Tetrahydropterin, which lacks a 6-position side chain, was one hundred times better than tetrahydropterin at entering the brain. Those data indicate that lipohilicity of the side chain, in general, is not an important determinant in brain penetration. The question remains as to whether nonresponding patients given tetrahydrobiopterin have a high enough brain concentration of tetrahydrobiopterin after therapy or whether there are other complicating factors.